This invention relates generally to gas turbine engines, and more specifically to vane sectors used in gas turbine engines.
At least some known gas turbine engines include, in serial flow arrangement, a fan assembly, a low pressure compressor, a high pressure compressor, a combustor, a high pressure turbine, and a low pressure turbine. The high pressure compressor, combustor and high pressure turbine are sometimes collectively referred to as the core engine. At least some known compressors include a plurality of rows of circumferentially-spaced rotor blades that extend radially outwardly from a rotor or disk. Adjacent rows of rotor blades are separated by a plurality of stator vane assemblies that are secured to the compressor casing. Each stator vane assembly includes a plurality of stator vanes, each of which includes an airfoil that extends between adjacent rows of rotor blades. At least some known stator vane assemblies include a plurality of stator vane segments that are circumferentially-joined together. Typically, the stator vane sectors are identical to each other, such that each stator vane sector spans an equal radial arc, and each vane sector includes an equal number of stator vanes.
Known airfoils have a series of natural frequencies associated with them. More specifically, each airfoil produces a wake in an air stream that is felt as a pulse by a passing airfoil. The combination of the number of stator vanes and the rotational speed of the compressor may coincide with a natural frequency of the rotor blades. The combination of the number of stator vane wakes (pulses) and the rotational speed of the compressor creates a stimulus that may coincide with a natural frequency of the rotor blades. Accordingly, in designing gas turbine engines, at least one design goal is to keep the majority of the airfoil natural frequencies outside of the designed engine operating range.
To reduce induced rotor blade vibrations, at least some known engines vary the vane spacing around the circumference of the engine casing to facilitate avoidance of rotor blade and stator vane natural frequencies or to reduce the amplitude of rotor blade resonant response at these frequencies. More specifically, within such designs the number of stator vanes is varied in one or more sectors of the stator vane assembly. Although the stator vane spacing may vary from one sector to the next, the stator vanes within each sector remain equally spaced relative to each other, and/or are designed with an equal pitch. The variation in vane spacing or pitch between stator vane sectors facilitates changing the frequency of the vane wakes to reduce the vibration response induced in adjacent rotor blades. However, as a result, circumferentially-spaced stator vane sectors are now different from each other and must be assembled in a certain relative order. Accordingly, the benefits derived from the variable or non-uniform stator vane spacing may be reduced or lost completely by misassembly of the stator vane sectors.